The present invention relates to an Improved Elastomeric Switch particularly adapted for low current switching applications, and for use in harsh environmental conditions.
Elastomeric Switch Assemblies generally comprise an elastomeric keypad mounted over a substrate having at least one pair of stationary contacts mounted thereon. Often such switch assemblies will include more than one switch, and the number of contact pairs formed on the substrate will correspond to the actual number of switches to be included in the assembly. The Elastomeric keypad is generally formed of a resilient material such as silicon rubber, and will include a number of raised tactile button actuators. Clearly, the number of actuators included in the keypad will correspond to the number of fixed contact pairs formed on the substrate. When assembled, the keypad is mounted over the substrate, with each tactile button is located over a fixed contact pair. Each individual switch within the assembly further includes a moving contact mounted between each corresponding tactile button and fixed contact pair. A particular switch is closed when its associated tactile button actuator is pressed downward toward the substrate. The tactile button actuator acts against the moving contact, forcing it against the fixed contact pair to create a conductive path through the switch. When the tactile button actuator is released, the resiliency of the elastomeric keypad lifts the actuator to it normal raised position, the moving contact is pulled away from the stationary contacts, and the circuit is opened.
Such elastomeric switch assemblies are well know and widely used. However in certain applications, including those applications where the switches are switching very low current loads, or are operating in particularly dirty environments, the performance of such switches has been inadequate. A major problem with the present generation of elastomeric switch assemblies has been contact bounce. Contact bounce is a phenomenon that results from the particular geometry of present day elastomeric switches. Generally, the tactile button actuator will act against the moving contact in a linear manner, pressing the moving contact at a right angle onto the fixed contacts. The moving contact engages the fixed contacts and closes the circuit as the tactile button actuator reaches the bottom of its stroke. Because the fixed contacts are formed on the rigid surface of the substrate, the moving contact has a tendency to "bounce" as it engages the two fixed contacts. This bouncing action has a tendency to rapidly open and close the switch circuit prior to the contacts settling into a fully closed position. This can result in false data signals being received at the logic circuitry charged with evaluating the state of the switch. Presently this problem has been resolved by taking multiple readings of the switch state in order to determine the validity of the switch state. This adds both complexity and cost to the evaluating circuitry.
Another problem with currently used elastomeric switch assemblies has been the buildup of contaminants on the switch contacts. This is an especially acute problem in applications where the switch must operate in a dirty environment filled with airborne contaminants. Opening and closing the switch will generally create a small amount of arcing which can fuse contaminants to the surface of the contacts. Over time, as the contaminants build up, the contact resistance of the switch increases. In low current switching applications such increased contact resistance can be extremely detrimental to the quality of the signal being conducted through the switch. In fact, with sufficient contact resistance the signal can be completely blocked. Therefore, an improved elastomeric switch is desired which will eliminate or greatly reduce contact bounce, and will prevent contaminants from building up on the surface of the contacts. Such a switch should allow contact overtravel beyond the point of initial contact engagement, with at least one switch contact being mechanically biased against another. Further, the switch contacts should be configured to provide a self cleaning operation wherein contaminants are removed from the contact surfaces with each operation of the switch in order to maintain a low switching resistance. Such a switch should be especially adapted for switching low current loads and be capable of extended use in environmentally harsh conditions.